Choice of fluid

The aim of a fluid challenge is to produce a small but significant (200 ml) and rapid increase in plasma volume. Colloid fluids are ideal, in that they give a reliable increase in plasma volume; crystalloids are rapidly lost from the circulation and larger volumes would be required to achieve an initial comparable increase of 200 ml in plasma volume. Furthermore, from Fig 1 it can be seen that colloid fluids may increase the capillary oncotic pressure p c and the reflection coefficient s, thus increasing the intravascular retention of plasma.

Where blood loss is the cause of hypovolemia, it would seem logical to use blood as the replacement fluid. However, while whole blood would achieve an adequate increase in circulating volume, the high hematocrit of packed red cells will correct red cell volume with less effect on plasma volume. Therefore it is appropriate to replace the plasma volume deficit with a plasma substitute. Since the effect of colloid on the reflection coefficient is determined by the combination of the molecular size and the effective capillary pore size, a lower-molecular-weight colloid (e.g. gelatin solution, low-molecular-weight hydroxyethyl starch, dextran) is recommended for short-term plasma volume expansion in simple hypovolemia and high-molecular-weight hydroxyethyl starch is recommended where there is a probability of capillary leak (We,,b,,b,,,,,,1,9,9,1).

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